linux-kernel - Re: [PATCH] mm/mempolicy: Fix decision-making issues for memory migration during NUMA balancing

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Message-ID: <Z0RgoOHMRFCTM1JB@tiehlicka>
Date: Mon, 25 Nov 2024 12:33:52 +0100
From: Michal Hocko <mhocko@...e.com>
To: Junjie Fu <fujunjie1@...com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, dave.hansen@...el.com
Subject: Re: [PATCH] mm/mempolicy: Fix decision-making issues for memory
 migration during NUMA balancing

On Sun 24-11-24 03:09:35, Junjie Fu wrote:
> When handling a page fault caused by NUMA balancing (do_numa_page), it is
> necessary to decide whether to migrate the current page to another node or
> keep it on its current node. For pages with the MPOL_PREFERRED memory
> policy, it is sufficient to check whether the first node set in the
> nodemask is the same as the node where the page is currently located. If
> this is the case, the page should remain in its current state. Otherwise,
> migration to another node should be attempted.
> 
> Because the definition of MPOL_PREFERRED is as follows: "This mode sets the
> preferred node for allocation. The kernel will try to allocate pages from
> this node first and fall back to nearby nodes if the preferred node is low
> on free memory. If the nodemask specifies more than one node ID, the first
> node in the mask will be selected as the preferred node."
> 
> Thus, if the node where the current page resides is not the first node in
> the nodemask, it is not the PREFERRED node, and memory migration can be
> attempted.
> 
> However, in the original code, the check only verifies whether the current
> node exists in the nodemask (which may or may not be the first node in the
> mask). This could lead to a scenario where, if the current node is not the
> first node in the nodemask, the code incorrectly decides not to attempt
> migration to other nodes.
> 
> This behavior is clearly incorrect. If the target node for migration and
> the page's current NUMA node are both within the nodemask but neither is
> the first node, they should be treated with the same priority, and
> migration attempts should proceed.

The code is clearly confusing but is there any actual problem to be
solved? IIRC although we do keep nodemask for MPOL_PREFERRED
policy we do not allow to set more than a single node to be set there.
Have a look at mpol_new_preferred

-- 
Michal Hocko
SUSE Labs